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SpringMVC中的DispatcherServlet请求分析

 更新时间：2024年01月05日 09:07:11   作者：it_lihongmin  
这篇文章主要介绍了SpringMVC中的DispatcherServlet请求分析, DispatcherServlet作为一个Servlet,那么当有请求到Tomcat等Servlet服务器时,会调用其service方法,再调用到其父类GenericServlet的service方法,需要的朋友可以参考下

一、service请求（servlet请求转换为Http请求）

DispatcherServlet作为一个Servlet，那么当有请求到Tomcat等Servlet服务器时，会调用其service方法。再调用到其父类GenericServlet的service方法，HttpServlet中实现，如下（开始请求的调用）：

@Override
public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException {
    HttpServletRequest  request;
    HttpServletResponse response;
    try {
        request = (HttpServletRequest) req;
        response = (HttpServletResponse) res;
    } catch (ClassCastException e) {
        throw new ServletException(lStrings.getString("http.non_http"));
    }
    service(request, response);
}

HttpServlet层：将request和response类型进行转换后，继续调用service方法：

protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
 
    String method = req.getMethod();
    
    if (method.equals(METHOD_GET)) {
        long lastModified = getLastModified(req);
        if (lastModified == -1) {
            // servlet doesn't support if-modified-since, no reason
            // to go through further expensive logic
            doGet(req, resp);
        } else {
            long ifModifiedSince;
            try {
                ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
            } catch (IllegalArgumentException iae) {
                // Invalid date header - proceed as if none was set
                ifModifiedSince = -1;
            }
            if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                // If the servlet mod time is later, call doGet()
                // Round down to the nearest second for a proper compare
                // A ifModifiedSince of -1 will always be less
                maybeSetLastModified(resp, lastModified);
                doGet(req, resp);
            } else {
                resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
            }
        }
 
    } else if (method.equals(METHOD_HEAD)) {
        long lastModified = getLastModified(req);
        maybeSetLastModified(resp, lastModified);
        doHead(req, resp);
    } else if (method.equals(METHOD_POST)) {
        doPost(req, resp);
    } else if (method.equals(METHOD_PUT)) {
        doPut(req, resp);
    } else if (method.equals(METHOD_DELETE)) {
        doDelete(req, resp);
    } else if (method.equals(METHOD_OPTIONS)) {
        doOptions(req,resp);
    } else if (method.equals(METHOD_TRACE)) {
        doTrace(req,resp);
    } else {
        String errMsg = lStrings.getString("http.method_not_implemented");
        Object[] errArgs = new Object[1];
        errArgs[0] = method;
        errMsg = MessageFormat.format(errMsg, errArgs);
        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
    }
}

根据调用的Http请求的方式，调用具体的底层（FrameworkServlet层）方法，get、post请求等都会有相同的处理,比如doGet如下：

@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    processRequest(request, response);
}
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
 
    long startTime = System.currentTimeMillis();
    Throwable failureCause = null;
 
    LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
    LocaleContext localeContext = buildLocaleContext(request);
 
    RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
    ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
 
    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
 
    initContextHolders(request, localeContext, requestAttributes);
 
    try {
        doService(request, response);
    } catch (ServletException | IOException ex) {
        failureCause = ex;
        throw ex;
    } catch (Throwable ex) {
        failureCause = ex;
        throw new NestedServletException("Request processing failed", ex);
    } finally {
        resetContextHolders(request, previousLocaleContext, previousAttributes);
        if (requestAttributes != null) {
            requestAttributes.requestCompleted();
        }
        logResult(request, response, failureCause, asyncManager);
        publishRequestHandledEvent(request, response, startTime, failureCause);
    }
}

主要的核心逻辑为doService，但是Tomcat是使用线程池的方式接受来自客户端的请求的，当前请求中可能带有Locate（国际化参数信息），那么需要使用ThreadLocal在请求前记录参数信息，在请求之后finally中将参数恢复回去，不会影响到下一个请求。

Spring经常会这样进行处理，比如AopContext等处理Aop切面信息。

二、doService请求（request中添加SpringMVC初始化的九大件信息）

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    logRequest(request);
 
    // Keep a snapshot of the request attributes in case of an include,
    // to be able to restore the original attributes after the include.
    Map<String, Object> attributesSnapshot = null;
    if (WebUtils.isIncludeRequest(request)) {
        attributesSnapshot = new HashMap<>();
        Enumeration<?> attrNames = request.getAttributeNames();
        while (attrNames.hasMoreElements()) {
            String attrName = (String) attrNames.nextElement();
            if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }
    }
 
    // Make framework objects available to handlers and view objects.
    request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
    request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
    request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
    request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
 
    if (this.flashMapManager != null) {
        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
        if (inputFlashMap != null) {
            request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
        }
        request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
        request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
    }
 
    try {
        doDispatch(request, response);
    }
    finally {
        if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Restore the original attribute snapshot, in case of an include.
            if (attributesSnapshot != null) {
                restoreAttributesAfterInclude(request, attributesSnapshot);
            }
        }
    }
}

1、打印请求日志

2、请求中添加属性（WebApplicationContext容器，i18n解析器，主题解析器，主题，重定向属性处理）

3、核心方法 doDispatch

三、doDispatch

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    HandlerExecutionChain mappedHandler = null;
    boolean multipartRequestParsed = false;
    // 异步请求属性解析（重定向）
    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    try {
        ModelAndView mv = null;
        Exception dispatchException = null;
        try {
            // 如果是Multipart上传文件请求，则调用multipartResolver.resolveMultipart（上传文件解析器进行解析）
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);
 
            // 确定Handler处理该请求（HandlerExecutionChain调用链，责任链模式）
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }
 
            // 根据初始化时加载的适配器挨个匹配是否能适配该调用链
            HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
 
            // 对请求头中的last-modified进行处理
            String method = request.getMethod();
            boolean isGet = "GET".equals(method);
            if (isGet || "HEAD".equals(method)) {
                long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                    return;
                }
            }
            // 对HandlerExecutionChain中的所有HandlerInterceptor调用preHandle方法
            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                return;
            }
 
            // 真正的请求调用
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
 
            if (asyncManager.isConcurrentHandlingStarted()) {
                return;
            }
            // 判断Controller返回的ModelAndView中是否有View，
            // 没有则使用viewNameTranslator执行没有试图的解析规则
            applyDefaultViewName(processedRequest, mv);
            // 对HandlerExecutionChain中的所有HandlerInterceptor调用postHandle方法
            mappedHandler.applyPostHandle(processedRequest, response, mv);
        } catch (Exception ex) {
            dispatchException = ex;
        } catch (Throwable err) {
            // As of 4.3, we're processing Errors thrown from handler methods as well,
            // making them available for @ExceptionHandler methods and other scenarios.
            dispatchException = new NestedServletException("Handler dispatch failed", err);
        }
        // 处理结果解析（ModelAndView或者Exception），保证最终会执行所以Interceptor的triggerAfterCompletion
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    } catch (Exception ex) {
        triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    } catch (Throwable err) {
        triggerAfterCompletion(processedRequest, response, mappedHandler,
                new NestedServletException("Handler processing failed", err));
    } finally {
        if (asyncManager.isConcurrentHandlingStarted()) {
            // 不知道什么情况下回进入相当于执行所以Interceptor的postHandle和afterCompletion方法
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        }
        else {
            // 清除当前文件上传请求的资源
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }
}

这里是整个SpringMVC的核心，每个流程都可能会比较复杂，后续单独分析。流程如下（主要是标红的步骤）：

1、异步请求属性解析（重定向）

2、如果是Multipart上传文件请求，则调用multipartResolver.resolveMultipart（上传文件解析器进行解析）

4、确定Handler处理该请求（HandlerExecutionChain调用链，责任链模式）

5、根据初始化时加载的适配器挨个匹配是否能适配该调用链

6、对请求头中的last-modified进行处理

7、对HandlerExecutionChain中的所有HandlerInterceptor调用preHandle方法

8、真正的请求调用

9、没有试图的解析返回

10、对HandlerExecutionChain中的所有HandlerInterceptor调用postHandle方法

11、处理结果解析（ModelAndView或者Exception），保证最终会执行所以Interceptor的triggerAfterCompletion

12、清除当前文件上传请求的资源

HandlerInterceptor方法调用

当我们需要使用Spring的拦截器时，会集实现HandlerInterceptor的preHandle、postHandle、triggerAfterCompletion方法。

当第4步完成后我们获取到了HandlerExecutionChain调用链，其中包括需要执行的拦截器和真正调用的方法（后面专门分析专门获取的）。

但是在上面的步骤看到了对拦截器的三个方法的调用时机，其实每个调用的方法都差不多，就看preHandle方法即可，调用HandlerExecutionChain的applyPreHandle方法如下：

boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HandlerInterceptor[] interceptors = getInterceptors();
    if (!ObjectUtils.isEmpty(interceptors)) {
        for (int i = 0; i < interceptors.length; i++) {
            HandlerInterceptor interceptor = interceptors[i];
            if (!interceptor.preHandle(request, response, this.handler)) {
                triggerAfterCompletion(request, response, null);
                return false;
            }
            this.interceptorIndex = i;
        }
    }
    return true;
}

由于preHandle方法允许返回false则不执行真实的Controller方法调用，所以需要每次判断。但是在执行preHandle和postHandle方法时，都允许最后调用一次triggerAfterCompletion方法。都是在拿到调用链中的所以有序的拦截器，轮训调用其对应的方法即可。

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